1. Field of the Invention
This invention relates to a liquid crystal display device.
2. Description of the Related Art
Conventionally, liquid crystal display devices are widely used as display devices of a television set or other electrical and electronic appliances etc. These liquid crystal display devices are requested to have display characteristics in which a displayed picture is stable and reliability of operation is maintained highly over a long period of time. One of the factors to determine whether display characteristic is good or bad is resistivity .rho. of liquid crystal material. In a liquid crystal display device which uses a liquid crystal material having small resistivity .rho., a very small electric current flows between electrodes opposing to each other across a liquid crystal material layer, so that the liquid crystal material is considerably deteriorated, reliability of operation is very low and stability of displayed picture is poor.
Particularly, in a TFT liquid crystal display device in which a plurality of picture element electrodes corresponding to a plurality of picture elements are arranged on one of opposing surfaces of a pair of substrates opposing to each other across a liquid crystal layer in a matrix pattern, a TFT (thin film transistor) is connected to each of these picture element electrodes and a voltage for changing an optical transmission factor of each picture element is selectively supplied to each electrode by turning ON and OFF this TFT, the following disadvantages are unavoidable. That is, when a predetermined voltage as video image data is applied to one picture element electrode by turning the TFT ON to thereby accumulate charges, the accumulated charges leak out so that the potential of the picture element electrode is fluctuated, thus optical transmission factor of the fluctuated picture element corresponding to the picture element electrode being fluctuated. As a result, a flicker occurs in a picture displayed on the TFT liquid crystal display device and a stability of picture is poor.
In order to solve this problem, it is proposed that the conventional liquid crystal display device uses a liquid crystal material whose resistivity .rho. is large.
However, in order to obtain a liquid crystal material having large resistivity, impurities must be substantially prefectly removed from liquid crystal material by repeating a refine-process over and over, so that the liquid crystal material having large resistivity becomes very expensive. Accordingly, the conventional liquid crystal display device using the liquid crystal material having large resistivity is high in manufacturing cost. Further, impurities adhere to a cell container into which liquid crystal is sealed, and it is substantially impossible to remove such impurities. Consequently, even if the liquid crystal having large resistivity is sealed in the cell container, it is difficult to maintain the resistivity of the liquid crystal high.
Accordingly, it is an object of the present invention to provide a liquid crystal display device which can substantially eliminate the aforenoted shortcomings and disadvantages encountered with the prior art. More specifically, it is an object of the present invention to provide a liquid crystal display device in which display of high image quality can be stably performed even if a liquid crystal material having a relatively small resistivity .rho. is utilized, and reliability can be maintained over a long period of time.